1. Field of the Invention
The present invention relates to a double monochromatic spectroscopic device, and more particularly to a wavelength calibrating mechanism and method of a pre-spectroscope and a main spectroscope in the double monochromatic spectroscopic device.
2. Description of the Related Art
FIG. 3 shows an exemplary wavelength scanning mechanism in a conventional double monochromatic spectroscopic device. A wavelength scanning mechanism 10 includes a pre-spectroscope 11 having a diffraction grating and a main spectroscope 12 having diffraction grating. An inlet slit 13 is arranged on the light-incident side of the diffraction grating of the pre-spectroscope 11. An intermediate slit 14 (which also serves as an inlet slit of the main spectroscope 12) is arranged on the light-outgoing side of the diffraction grating of the pre-spectroscope 11. An outlet slit 15 is arranged on the light-outgoing side of the main spectroscope 12. Light from a light source, after having passed through the inlet slit 13, is first incident on the diffraction grating of the pre-spectroscope 11. The incident light is diffracted by the diffraction grating, and only the monochromatic light at a predetermined wavelength of the diffracted light passes through the slit 14. A main mirror 16 is arranged in front of the slit 14. The monochromatic light passed through the slit 14, after reflected from the main mirror 16, is incident on the diffraction grating of the main spectroscope 12. As described later, the diffraction grating of the main spectroscope 12 is in synchronism with that of the pre-spectroscope 14. For this reason, the monochromatic light at the wavelength equal to that of the monochromatic light outgoing from the outlet slit 14 of the pre-spectroscope 11 outgoes from the diffraction grating of the main spectroscope 12 and reflects from the main mirror 16. Thereafter, the reflected monochromatic light passes through the outlet slit 15. The monochromatic light passed through the outlet slit 15 serves as output light of the wavelength scanning mechanism 10 and is projected onto a sample.
The wavelength of the output light of the wavelength scanning mechanism 10 varies in such a manner that the diffraction grating of the pre-spectroscope 11 and that of the main spectroscope 12 rotate in synchronism with each other, thereby performing wavelength scanning. The driving mechanism for this wavelength scanning includes a single feeding screw 17 and two sign bars 18, 19 as main components and is structured such that the pre-spectroscope 11 and the main spectroscope 12 are connected by a parallel link 20 (This structure is referred to as a feeding screw/sign bar system).
However, this feeding screw/sign bar system requires a large number of components. Particularly, in order to realize the parallel link 20 with high rigidity and high precision for rotating the diffraction grating of the pre-spectroscope 11 smoothly, this feeding screw/sign bar system requires a large number of components. Further, the accuracy necessary for the synchronizing operation between the pre-spectroscope 11 and the main spectroscope 12 cannot be assured by controlling a motor 21 which is a driving source. For this purpose, the two sign bars 18 and 19 must be adjusted strictly. As a result, the operation for adjustment becomes troublesome.
In order to obviate such an inconvenience, there have been proposed a spectroscopic device with a synchronizing mechanism which includes individual driving units (first driving unit and second driving unit) attached to diffraction elements of the pre-spectroscope and the main spectroscope and a control unit for operating both driving units in synchronism with each other by sophisticated control (JP-A-8-136344). This synchronizing control controls the non-linearity between the rotating angle of each of the diffraction elements of the pre-spectroscope and the main spectroscope and the wavelength of the outgoing light from each diffraction element, and also controls the reduction gear ratio of the first driving unit and that of the second driving unit.
In the spectroscopic device equipped with the synchronizing mechanism, the number of components necessary to realize the wavelength scanning mechanism is reduced as compared with the conventional feeding screw/sign bar system. For this reason, the entire device has been downsized and its reliability has been improved. In addition, since there is no mechanical link between both spectroscopes, advantages of increasing the degree of freedom of arrangement of both spectroscopes and selection of gratings and mounting manners have been obtained.
However, the spectroscopic device such a sophisticated control mechanism also requires that the wavelength of each spectroscope is calibrated after the device has been assembled or has been used for a long time. The wavelength of the light extracted by the spectroscope is determined by the arrangement of the inlet slit, outlet slit and spectroscopes (diffraction coefficients) therebetween. The assembling error occurring when these components are assembled leads to an error in the wavelength of the diffracted and extracted light. Further, the accuracy of setting the angle of the driving source for each spectroscope and the precision of the spectroscopes also affect the accuracy of the wavelength of the extracted light. In a double monochromatic spectroscopic device, changes in the wavelength of each of the pre-spectroscope and the main spectroscope greatly deteriorate the extraction efficiency of light energy. The above various errors greatly affect the performance of the spectroscopic device.
The above JP-A-8-136344 discloses the structure of the spectroscopic device, but does not teach the calibration of the wavelength.